This invention relates to oil well drilling, particularly, to a downhole cementing tool assembly for directing a slurry of cement and water to a point down hole for cementing the well casing in the well bore.
Many related operations are involved to drill and complete a well. Initially, a casing program is developed on the basis of geophysical information relating to the drill site. The casing program typically provides for the installation of various sizes and types of casing to meet the objective of the drilling program. Upon completion of the drilling program and other preparatory activity, drilling of the well hole is begun. At various stages during the drilling process, casing is set in the well and cemented in place. Typically, a short string of pipe, conductor pipe, is set in the well. The conductor pipe extends from the surface a short distance into the formation. The conductor pipe is particularly useful where the ground is soft, as for example, in a swamp, marsh or offshore location. The conductor pipe may or may not be cemented in place as required. As drilling progresses, casing is set to protect the well. Casing prevents the contamination of fresh water in the upper zones of the formation being drilled and prevents water from entering the well from the surrounding formations. In addition, casing confines production of the well fluids to a particular formation or zone and provides a means of controlling formation pressure.
In a typical drilling program, as is well known in the prior art, various sizes of casing are set and cemented in the well bore. Surface casing, for example, has a smaller diameter than the conductor pipe and is set concentrically within the conductor pipe. The surface casing is set deep enough to protect the well from cave in of loose formations which are often encountered near the surface of the formation being drilled. Intermediate casing, set concentrically within the surface casing, seals off weak zones encountered during drilling to prevent loss of circulation, contamination of drilling fluid and to seal off producing zones which may exist above the bottom of the well. Production casing is the last casing set in the well. As each type of casing is set in the well, cement, usually in the form of a slurry comprising cement and water, is pumped down the casing and into the well bore. The cement is forced up around the external surface of the casing and permitted to set before drilling to further depths is commenced.
After the cement has set, the bottom of the well bore is completely sealed with the cement. Some cement extends up into the lower portion of the casing and must be subsequently drilled through upon continuation of drilling the well to a deeper depth. This process is continued with different size casing set and cemented at various depths in the formation. Upon reaching the production depth, production casing is set and cemented in the well bore. The production casing is perforated in the producing zone permitting the formation fluids to enter the production tubing and either flow to the surface or lifted to the surface in a known manner.
In the drilling of a well, drilling fluid is used to control or offset the formation pressures encountered during drilling of the well. Another important use of the drilling fluid is to remove the drill cuttings from the bottom of the well bore. Thus, during drilling, the well bore is filled with drilling fluid. While setting casing in the well bore, it is desirable to prevent the drilling mud from entering the casing while it is being lowered in the well. Typically, a float collar is inserted one or two joints above the bottom of the casing string, which float collar includes a check valve at the lower end thereof which permits fluid to pass downward through the casing but prevents fluid from passing upward into the casing. The float collar has the additional benefit of closing the bottom end of the casing string so that it floats or is buoyed in the drilling fluid during the decent of the casing string in the well bore, thereby decreasing the load upon the derrick. The float collar also prevents the back flow of cement during the cementing operation.
As the casing string is lowered in the well bore, the drilling fluid is displaced and forced out of the well bore. The drilling fluid flows out of the well bore in the annular space formed between the outside of the casing and the inner wall of the well. The casing string is empty above the float collar and once positioned in the well bore, a cement slurry is pumped into the casing. A plug is then positioned in the casing above the cement slurry and drilling fluid is pumped into the casing forcing the plug downward to compress or push the cement slurry through the bottom end of the casing string. Recall that the check valve in the float collar permits the cement slurry to pass downward through the casing out the lower end of the casing and up around the outside of the casing to thereby fill the annular space and cement the casing in the well bore. After the cement has set, the cement plug and float collar are drilled out and drilling of the well continues.
The above discussion is illustrative of drilling and cementing operations well known in the prior art. A problem encountered after the cementing operation has been completed, is removal of the cementing tool assembly so that drilling the well may be continued. The cementing tool assembly (wiper plug and cementing plug) is fabricated of frangible material, typically some type of resilient material such as rubber and is usually drilled out. However, since the wiper plug and cementing plug are loose in the casing above the float collar, they tend to rotate with the action of the rotating drill bit so that some difficulty is encountered in drilling through the cementing tool assembly. The cost of drilling is very expensive and therefore, it is desirable to drill through the cementing tool assembly as quickly as possible so that drilling the well may be continued with the least possible interruption.
The apparatus of the present invention overcomes the disadvantages associated with the prior art cementing tool assemblies by providing a locking mechanism on the lower end of both wiper plug and the cementing plug to prevent rotation of the cementing tool assembly and wiper plug within the well bore. In addition, the wiper plug and cementing plug of the present invention incorporate frangible internal cutters to aid in the breakdown of the cementing tool assembly so that it is quickly drilled through. The apparatus of the present invention overcomes these and other problems encountered when cementing casing of a well bore.